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Modeling Atmospheric O2 Over Phanerozoic Time Geochimica et Cosmochimica Acta, Vol. 65, No. 5, pp. 685–694, 2001 Copyright © 2001 Elsevier Science Ltd Pergamon Printed in the USA. All rights reserved 0016-7037/01 $20.00 ϩ .00 PII S0016-7037(00)00572-X Modeling atmospheric O2 over Phanerozoic time R. A. BERNER* Department of Geology and Geophysics, Yale University, New Haven, CT 06520-8109, USA (Received May 18, 2000; accepted in revised form October 9, 2000) Abstract—A carbon and sulfur isotope mass balance model has been constructed for calculating the variation of atmospheric O2 over Phanerozoic time. In order to obtain realistic O2 levels, rapid sediment recycling and O2-dependent isotope fractionation have been employed by the modelling. The dependence of isotope fractionation on O2 is based, for carbon, on the results of laboratory photosynthesis experiments and, for sulfur, on the observed relation between oxidation/reduction recycling and S-isotope fractionation during early diagenetic pyrite formation. The range of fractionations used in the modeling agree with measurements of Phanerozoic sediments by others. Results, derived from extensive sensitivity analysis, suggest that there was a positive excursion of O2 to levels as high as 35% during the Permo-Carboniferous. High O2 at this time agrees with independent modeling, based on the abundances of organic matter and pyrite in sediments, and with the occurrence of giant insects during this period. The cause of the excursion is believed to be the rise of vascular land plants and the consequent increased production of O2 by the burial in sediments of lignin-rich organic matter that was resistant to biological decomposition. Copyright © 2001 Elsevier Science Ltd 1. INTRODUCTION 2000). Most of this work has focused on the control of organic production in the oceans by nutrient elements such as phos- The evolution of atmospheric oxygen over geologic time has phorus or nitrogen. However, in these studies calculations of been a subject of major interest to both the earth and biologic O levels as a function of time were not constrained by estimi- sciences. This is because O is both a principal cause and 2 2 nation of actual fluxes of both carbon and sulfur as they affect principal effect of biologic evolution. On shorter time scales the input and output of O to and from the atmosphere. atmospheric oxygen concentration represents the balance be- 2 The calculation of past atmospheric oxygen concentrations is tween production by photosynthesis and consumption by res- possible using the approach pioneered by Garrels and Lerman piration. On a long geologic time scale the level of O is 2 (1984). This method uses the isotopic record of carbon and affected by both geological and biologic processes. It is pro- sulfur in sedimentary rocks to calculate the rates of the pro- duced by net photosynthesis (photosynthesis minus respira- cesses that affect O on long time scales, mainly burial in tion), represented by the burial of organic matter and pyrite 2 marine sediments and weathering on land of organic matter and (FeS ) in sediments, and it is consumed by the oxidation of old 2 biogenic pyrite. Using the Garrels and Lerman model, esti- buried sedimentary organic matter and pyrite upon uplift and mates of the rates of weathering and burial, and their effect on exposure to weathering on the continents. The oxidation of O , have been made by Berner (1987), Francois and Gerard reduced gases produced by the thermal decomposition of or- 2 (1986), Kump and Garrels (1986), Kump (1989), Lasaga ganic matter and pyrite at great depths also exerts a control on (1989) and Petsch and Berner (1998). However, none of these O . (The formation and reduction of iron oxides have only a 2 studies has presented a realistic history of O variations over minor effect on O —Holland, 1978). 2 2 Phanerozoic time. This is because a key difficulty in this type Although much attention has been given to the rise of O and 2 of modelling is formulating negative feedback mechanisms life during the Precambrian (e.g., Berkner and Marshall, 1965; consistent with both geologic and biologic processes and the Cloud, 1976; Walker, 1977; Schidlowski, 1984; Holland, 1984; isotopic record of carbon and sulfur. Without feedback the Des Marais et al.,1992; Canfield and Teske, 1996), little atten- modelling leads to calculated variations in O over Phanerozoic tion has been paid to its continued evolution during the Pha- 2 time that are biologically and physically impossible, such as nerozoic. Some previous studies (Berner and Canfield, 1989) large negative values for O concentration during the early have suggested that atmospheric oxygen concentrations may 2 Paleozoic. have changed appreciably over the Phanerozoic whereas others Initial attempts to include negative feedback in carbon-sulfur (e.g., Watson et al., 1978; Lenton and Watson, 2000) have models have let the rate of organic matter and pyrite weathering suggested that O has been held essentially constant by strong 2 be a direct function of the concentration of atmospheric O .In negative feedbacks. 2 other words as O rises, the rate of its consumption by weath- Recently there has been a lot of interest in negative feedback 2 ering also rises. However, this approach, although it brings mechanisms and how they might have controlled Phanerozoic about somewhat damped variations in O , still results in im- O levels (Holland, 1994; Van Cappellen and Ingall, 1997; 2 2 possible O histories (Lasaga, 1989; Berner and Petsch, 1998). Falkowski, 1997; Colman et al., 1998; Lenton and Watson, 2 With O2-dependent weathering, positive feedback is unavoid- ably introduced because of the need for carbon and sulfur *Author to whom correspondence should be addressed isotope mass balance. (For example, if organic C burial sud- ([email protected]). denly increased, atmospheric O2 would increase and organic C 685 686 R. A. Berner weathering would also increase if O2-dependent weathering (Canfield and Teske, 1996) that, in the presence of O2, sedi- were present. Although this would help to lower atmospheric mentary sulfur undergoes increased isotope fractionation be- O2, it would also lead to an increased input to the ocean of light cause of multiple cycles of sulfate reduction and sulfide oxi- carbon from the extra organic weathering necessitating, for dation accompanying the overturn of sediments near the steady state, futher increased organic C burial and O2 produc- sediment/seawater interface due to bioturbation and wave and tion—in other words, positive feedback). In addition, doubt has current stirring. As atmospheric O2 increases, it is assumed been placed on whether the rate of O2 uptake by weathering is that, on a global average basis, the extent of oxidation/reduc- actually related to atmospheric O2 level (Holland, 1978; Hol- tion cycling increases which results in an increase in overall land, 1984; Chang and Berner, 1999). Unavoidable positive isotope fractionation between seawater sulfate and the pyritic feedback is also introduced if the rate of burial of organic sulfur ultimately buried in sediments. matter and/or pyrite are assumed to be an inverse function of From the studies of processes controlling O2, it is reasonable atmospheric O2. to assume that calculated past O2 levels should lie within One way to add unencumbered negative feedback to Garrels certain extremes. In the present paper limits are defined by the and Lerman type isotope modelling is to introduce rapid recy- response of forest fires to changing O2. At sufficiently low O2 cling (Berner, 1987). In rapid recycling (see also, Berner and concentration forest fires will not take place, but there is evi- Canfield, 1989) it is assumed that younger rocks that have been dence of fires, in the form of fossil charcoal (Chaloner, 1989), deposited more recently are weathered preferentially. In this for all periods since the rise of woody plants about 425 million way rapid variations in the isotopic composition of sediments years ago. On the other hand, O2 levels could not have been so as they are buried do not result in equally large complementary high that the destruction of all terrestrial life by fires took place variations in oceanic isotopic composition because the just- (Watson et al., 1978; Kump, 1989; Robinson, 1989). From deposited rocks are soon weathered and the anomalous isotopic these studies and our earlier modeling study (Berner and Can- composition rapidly returned to the ocean. However, calcula- field, 1989) we have adopted the range 10 to 40% O2 as a tions show that this approach alone is insufficient to produce reasonable outside limit for the testing of our model calcula- realistic O2 levels and something more is needed. tions. A mathematical adjustment to Garrels and Lerman-type In the present paper calculations of the concentration of O2 modelling, that mimics negative feedback while conserving over Phanerozoic time are done using the Garrels and Lerman carbon and sulfur mass balance, is to let the fractionation of model, modifed to accommodate rapid recycling and O2-de- isotopes during organic matter and pyrite formation be a func- pendent C and S isotope fractionation. By sensitivity analysis, tion of the level of atmospheric O2. This idea is reasonable. the relative importance of each modification is illustrated. From 12 Plant biomass is enriched in C relative to CO2 due to signif- this analysis, curves of O2 vs. time can be obtained that are in icant isotope discrimination during photosynthesis (e.g., Far- agreement with laboratory measurements of isotope fraction- quhar et al., 1982). If rates of plant respiration (photorespira- ation and that agree with independent calculations based on the tion) increase in response to elevated O2/CO2, then less rubisco abundance of organic carbon and pyrite sulfur in sedimentary is available for CO2 fixation and as a result CO2 builds up in the rocks (Berner and Canfield, 1989).
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